Screw Compressors

Rotary screw compressors are the most commonly used in industrial applications due to their efficiency, reliability, and continuous operation capability.

Operating Principle

The screw compressor uses two helical rotors (male and female) that rotate in opposite directions inside a housing.

How They Work

1. Suction: Air enters through the suction side when the rotor lobes separate
2. Trapping: The rotors trap air in pockets between the lobes and housing
3. Compression: As the rotors turn, the pockets become smaller
4. Discharge: Compressed air exits through the discharge port

Cross-section showing male and female helical rotors. Air enters through suction (left), is progressively compressed between the lobes, and exits through discharge (right).

Internal Compression Ratio

Each screw unit has a fixed internal compression ratio, determined by rotor and port geometry:

$$CR_{internal} = \frac{V_{suction}}{V_{discharge}}$$

Pressure Matching

If system pressure doesn't match the internal CR, losses occur:

• Over-compression: Internal CR > system pressure (energy waste)
• Under-compression: Internal CR < system pressure (backflow and noise)

Screw Compressor Advantages

Feature	Benefit
Continuous flow	No pulsations, no large tank required
High duty cycle	100% continuous operation
Low noise	65-75 dB typical
Long lifespan	40,000-100,000+ hours
Simple maintenance	Predictable intervals
Compact	High power in small footprint

Disadvantages

Feature	Consideration
Initial cost	Higher than piston for small capacities
Part-load efficiency	Lower without VSD
Contaminant sensitivity	Requires inlet filtration
Oil system complexity	In lubricated models

Screw Compressor Classification

Screw Compressors
├── By lubrication
│   ├── Oil-injected (lubricated)
│   └── Oil-free
│
├── By number of stages
│   ├── Single stage (up to ~190 psi / 13 bar)
│   └── Two stage (up to ~360 psi / 25 bar)
│
└── By capacity control
    ├── Load/Unload (on/off)
    ├── Modulation (inlet valve)
    ├── Variable Speed (VSD)
    └── Hybrid

Typical Applications

Application	Recommended type
General industrial	Lubricated, fixed speed
Variable demand	Lubricated with VSD
Food and pharmaceutical	Oil-free
High pressure (>190 psi)	Two stage
Continuous 24/7 operation	Lubricated or oil-free per quality needs